1. Field of the Invention:
This invention relates to methods and apparatus for detecting leaks in liquid storage vessels. More particularly, this invention relates to a method and apparatus for measuring changes in the level of a liquid surface in a storage vessel by splitting a laser beam, reflecting the split beams off of a reference surface and the liquid surface, recombining the reflected beams and analyzing the recombined beam to detect changes in the beam's fringe patterns.
2. Description of the Prior Art:
The detection of leaks in liquid storage vessels is a significant problem, particularly when the storage vessels are buried underground. Buried steel tanks are commonly used to store gasoline and chemicals. When these tanks come into contact with the soil, electrolytically enhanced corrosion may occur resulting in the formation of pin hole size holes, larger leakage holes, and/or seapage areas in the tanks. In addition, these and other types of storage vessels can suffer from weld splits, cracks, fractures, etc., all of which may contribute to leakage either out of or into the storage vessel. Liquid loses from storage tanks are not only economically undesireable, but may result in substantial damage to the environment, particularly when the stored liquid is a petroleum product or potentially toxic chemical. Leakages on the order of 0.05 gallons per hour are frequently of concern. In a storage vessel having a capacity on the order of 8,000 gallons, however, a leakage rate of 0.05 gallons per hour may result in a liquid level drop on the order of only about 0.001 inches per hour, depending on the tank geometry and liquid level. Therefor, high resolution measuring techniques are needed to adequately measure storage vessel leakage rates of this order of magnitude.
One prior method used to estimate leakage losses from underground gasoline storage tanks employs a stand pipe fixed to the tank fill hole. The storage tank and the stand pipe are filled with gasoline and then the amount of additional gasoline required to maintain the gasoline at a predetermined level in the stand pipe is measured over a relatively long period of time, usually on the order of about eight hours. The time required to use this method makes leakage measurement an inconvenient and costly process.
Other prior methods have employed the use of a manometer to estimate leakage loss rates. These methods, however, lack the resolution required to accurately measure relatively small leakage losses in large storage vessels.
It is one object of the invention to provide a high resolution leakage rate detection method and apparatus capable of detecting and measuring leakage rates in large storage vessels on the order of 0.05 gallons per hour.
It is a further object of the invention to provide a method and apparatus for detecting and measuring leakage rates from storage vessels in a relatively short period of time, such as one hour or less.
It is a further object of the invention to provide a method and apparatus for measuring leakage rates from storage vessels which are not dependent upon the liquid level in the storage vessel and which does not require filling of the storage vessel prior to measurement of leakage rates.